With the popularization of portable telephones in recent years, demand is increasing for speech communication and data communication that uses mobile stations such as portable telephones not only outdoors but indoors as well. To meet this increasing demand, ultra-compact base stations are being developed that can be set up indoors, such as in users' residences or small-scale offices. Because the cells of these base stations are extremely small compared to the cells of base stations that are set up outdoors, they are referred to as femtocells. These ultra-compact base stations are called femto base stations.
As the operating form of these femto base stations, forms have been proposed in which only pre-registered mobile stations are able to connect to the femto base stations and perform communication.
FIG. 1 is a block diagram showing a wireless communication system that includes a femto base station.
In FIG. 1, femto base station 101 forms femtocell 102 and connects to network 105 by way of femto-GW (Gateway) 104. Mobile station 103-1 is registered in femto base station 101 and is permitted to connect with femto base station 101. Mobile station 103-2 is not registered in femto base station 101 and is not permitted to connect with femto base station 101. In the following explanation, mobile stations that are registered in femto base station 101 including mobile station 103-1 are also referred to as registered mobile stations (or registered wireless stations). In addition, mobile stations that are registered in femto base station 101 including mobile station 103-2 are also referred to as nonregistered mobile stations (or nonregistered wireless stations).
When in femtocell 102 of femto base station 101, a registered mobile station is able to connect with network 105 by way of femto base station 101.
Apart from this case, a base station in a mobile communication system transmits pilot signals within the cell of its own base station. Upon receiving these pilot signals, a mobile station transmits, for example, a connection request signal to the base station, carries out processes such as establishing synchronization and estimating channels, and connects to the base station. As a result, the base station, by taking action such that the mobile station is able to receive pilot signals with good quality, is able to offer good communication quality. The situation is the same in a femto base station.
In a mobile communication system, the radio wave (desired wave) of a mobile station that is the communication destination of a base station may receive interference from the radio waves of other mobile stations. To suppress this type of interference, a directional antenna may be applied in the transmission/reception antenna in the mobile communication system to raise the circuit capacity of the mobile communication system.
As a directional antenna, an adaptive array antenna has been proposed in which a plurality of antenna elements are aligned in array form that can dynamically alter directivity in concert with changes in the radio-wave environment. In an adaptive array antenna, the signal from each antenna element undergoes a weighting process that controls amplitude and phase and is then synthesized to enable directing the point at which the directivity is maximum (main beam) in the direction of the desired wave or directing the point at which the directivity falls (null) in the direction of interference waves.
The technology that enables this dynamic alteration of directivity matched to changes in the radio-wave environment includes the base station apparatus described in Patent Document 1.
In this base station apparatus, cells are divided into a plurality of sectors. The base station apparatus detects traffic, which is the number of mobile stations present in each of these sectors. The base station apparatus increases directivity in the direction of the sectors having greater traffic. Because directivity can thus be increased in the direction in which there are more desired waves, interference can be suppressed.
In contrast, in a femto base station, a non-directional antenna is normally applied such that one room can be covered by one apparatus.
The use of a femto base station in a W-CDMA (Wideband Code Division Multiple Access) or E-UTRA (Evolved Universal Terrestrial Radio Access) system has been proposed. E-UTRA is referred to as LTE (Long Term Evolution).
When a femto base station is used in W-CDMA, data transmission that uses individual channels that are subjected to transmission power control in uplink circuits and downlink circuits and data transmission that uses common channels in downlink circuits are carried out as described in 3GPP TS 25.214 V7.3.0. When a femto base station is used in E-UTRA, the wireless frequency band is divided into a plurality of resource blocks (PRB: Physical Resource Block) as described in 3GPP TS 36.300 V8.1.0. The scheduler provided in the femto base station then assigns the resource blocks and data transmission is carried out using these assigned resource blocks.